Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 issued to Creux discloses a basic construction of a scroll type fluid displacement apparatus which includes a pair of scrolls, each having a circular end plate, and a spiroidal or involute spiral element affixed to or extending from one axial end surface of each circular end plate. These scrolls are maintained angularly and radially offset so that both spiral elements interfit and make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets changes. Since the volume of fluid pockets increases or decreases, dependent on the direction of the orbital motion, the scroll type fluid displacement apparatus is applicable to compress, expand or pump fluids.
Generally, in conventional scroll type fluid displacement apparatus where the orbiting scroll is supported on a crank pin in a cantilever manner, axial slant occurs. Axial slant also occurs because the movement of the orbiting scroll is not rotary motion around the center of the orbiting scroll, but is orbital motion caused by the eccentric movement of a crank pin driven by a drive shaft. Several problems result because of this axial slant including improper sealing of line contacts, vibration of the apparatus during operation and noise caused by the physical striking of the spiral elements. One simple and direct solution to these problems is the use of a thrust bearing device for carrying the axial loads. Thus, scroll type fluid displacement apparatus usually have a thrust bearing device within its housing.
Referring to FIGS. 1, 2 and 3, one recent attempt to improve a rotation preventing and thrust bearing device in scroll type fluid displacement apparatus is illustrated. FIG. 1 is a vertical sectional view of a part of a compressor; FIG. 2 is an exploded perspective view of a rotation preventing/thrust bearing device; and FIG. 3 is a diagrammatic front view of the rotation preventing/thrust bearing device, illustrating the manner in which rotation is prevented.
A rotation preventing/thrust bearing device 37' surrounds a boss 273' of orbiting scroll 27' and includes an orbital portion, a fixed portion and bearings, such as a plurality of balls. The fixed portion includes (1) an annular fixed race 371' having one end surface fitted against an axial end surface of an annular projection 112' of a front end plate 11', and (2) a fixed ring 372' fitted against the other axial end surface of fixed race 371' to extend outwardly therefrom and cover the other end surface of fixed race 371'. Fixed race 371' and ring 372' are attached to the axial end surface of annular projection 112' by pins 373'. The orbital portion also includes (1) an annular orbital race 374', which has one end surface fitted against an axial end surface of a circular end plate 271', and (2) an orbital ring 375' fitted against the other axial end surface of orbital race 374' to extend outwardly therefrom and cover the other axial end surface of orbital race 374'. A small axial clearance is maintained between the end surface of fixed ring 372' and the end surface of orbital ring 375'. Orbital race 374' and ring 375' are attached to the end surface of circular end plate 271' by pins 376'.
Fixed ring 372' and orbital ring 375' each have a plurality of holes or pockets 372a' and 375a' in the axial direction, with the number of holes or pockets in each ring 372' and 375' being equal. The holes or pockets in each ring 372' and 375' being equal. The holes or pockets 372a' of fixed ring 372' correspond to or are a mirror image of the holes or pockets 375a' on orbital ring 375', i.e., each pair of pockets facing each other have the same size and pitch, and the radial distance of pockets from the center of their respective rings 372' and 375' is the same, i.e., the centers of pockets are located at the same distance from the center of rings 372' and 375'. Thus, if the center of the rings 372' and 375' were aligned, which they are not in actual operation of the rotation preventing/thrust bearing means 37', the holes or pockets 372a' and 375a' would be identical or in alignment. Bearing elements, such as balls 377' are placed between facing or generally aligned pair of pockets 372a' and 375a' of fixed and orbital rings 372' and 375'.
In this arrangement, if the orbiting scroll is driven by the rotation of the drive shaft in the direction indicated by arrow A in FIG. 3, the center of orbital ring 375' orbits about a circle of radius Ror (together with orbiting scroll 27'). However, a rotating force, i.e., moment, which is caused by the offset of the acting point of the reaction force of compression and the acting point of the drive force, acts on orbiting scroll 27'. This reaction force tends to rotate the orbiting scroll 27 in the clockwise direction about the center of orbital ring 375'. In the embodiment as shown in FIG. 3, eighteen balls 377' are placed between the pockets 372a', 375a'. In the positions shown in FIG. 3, the interaction between the nine balls 377' in the top half of the rotation preventing/thrust bearing device and the edges of pockets 372a', 375a' prevents the rotation of orbiting scroll 27'. The magnitude of rotation preventing forces for the top half of the rotation preventing/thrust bearing device are shown by force vectors fc.sub.1 - fc.sub.5. In any given position of orbiting scroll 27' and orbital ring 375' about the orbital radius Ror, only half the balls 377' and pockets 372a', 375a' function to prevent rotation of the orbiting scroll 27', and to varying degrees as illustrated by the magnitude of force vectors fc.sub.1 -fc.sub.5 ; however, all the balls carry the axial thrust load from orbiting scroll 27'. Therefore, this rotation preventing/thrust bearing device includes a large number of balls, which is desirable for carrying the thrust load from the orbiting scroll. However, in the assembly process of the compressor, each of the balls must be placed in each pocket and between the edges of facing pockets. Thus, assembly of the compressor can be complicated and time consuming.